Sputtering techniques have been developed for coating long flexible sheets of substrate with thin film deposits of desired metal-based materials. The process requires a cathode, an anode, a gas atmosphere for establishing a gas-discharge plasma, a source of sputtering material, an evacuable chamber to house these items, a power supply, a gas supply, a supply of the substrate to be coated, and apparatus for passing the substrate adjacent the plasma chamber. Such an apparatus which deposits a plurality of layers onto a substrate is described in U.S. Pat. No. 4,298,444, entitled "Method for Multilayer Thin Film Deposition".
The evacuable chamber may be evacuated to a pressure as low as a few millitorr in some systems. An electric field of several hundred volts is established between the cathode and anode to produce glow discharge or plasma. The substrate is positioned within a few inches of the cathode. The cathode is usually formed of the metal to be sputtered and is referred to generally as the target. Ions from the plasma are accelerated toward the cathode by the electric field where they strike and transfer their kinetic energy to atoms of metal, causing them to be ejected as a hot gas. This metal gas condenses on and coats the substrate.
If the sputtering process takes place in an inert gas, such as argon, that does not react with the atoms of the target, the coating deposited on the substrate will remain relatively pure. However, the mixture of a normally reactive gas, such as oxygen or fluorine, with the nonreactive gas changes the composition, and therefore, the characteristics, of the sputtered deposit. A general term which may be used to describe this is MO.sub.x, where M represents the metal, O represents oxygen, and the subscript x represents the amount of oxygen which combines with the metal. The electrical properties of the resultant sputtered deposit are often important. As a general rule, where the metal deposited is electrically conductive, the less oxygen that combines with it the more conductive is the deposited coating. Conversely, the more oxygen, the less conductive it is.
Recent uses of thin film coatings on sheets of flexible material have developed in the area of optics, and more specifically, the area of window manufacture to control the frequencies of light which pass through. It is becoming more common to construct such windows with a sheet of a flexible material, such as plastic or polymer.
One of the fundamental problems in the manufacture of such sheets is in obtaining consistency in the thickness or other characteristic of the deposited layer(s). Conventionally, a deposition chamber is used with a sensor placed downstream to measure the desired characteristic. Control is then provided by a feedback loop to vary one or more of the input variables. This has resulted in a reasonable consistency along the length of the substrate but did not affect the inconsistencies which resulted across the width of the substrate. It has been found that because of the dynamics of the operation of the plasma chamber, even with attempted uniform construction of the plasma chamber structure along the width of the substrate, there is significant fluctuation of the plasma.
This has been improved by the application of gases to the chamber in individual zones with feedback localized to each zone. This has done much to improve the lateral consistency of the resulting sputtered layer. However, need still exists to provide for more uniform consistency laterally across the substrate.